An insulated gate bipolar transistor (IGBT) can be used as a switch in an apparatus, such as a motor drive or an inverter. The IGBT has three electrodes or terminals referred to as a gate, an emitter, and a collector that is sometimes called a drain. The potential applied to the gate controls the turn-on and turn-off operations of the transistor. As switches, IGBTs offer relatively high efficiency and fast switching capabilities compared to other switching devices.
An IGBT is often used to switch loads driven from several hundred to a few thousand volts. In such an installation, a failure may occur if the load is shorted when the transistor is fully turned on and carrying a large current. The IGBT then is subjected to an excessive power dissipation condition due to the high current flow through the device and the rapidly rising voltage across the IGBT's emitter and collector. If the short circuit condition exists for a sufficient period of time, the device can fail due to the excessive power dissipation. It can be beneficial, therefore, to turn off the IGBT under such a short circuit load condition. Because of the presence of parasitic inductance in the collector and emitter circuits of the IGBT, the maximum rate of turn off of the collector current may be restricted so as to not exceed the maximum collector-emitter rating of the device.
As a result, it is common practice to monitor the voltage between the collector and emitter of the IGBT device and provide a feedback signal indicating whether the differential voltage is large or small. The feedback signal is supplied to circuitry, such as a controller, that controls the bias of the gate and thus controls the operation of the transistor.
Previous circuits for monitoring IGBT devices simply connected a diode to the collector, wherein the diode was reverse biased when the collector was at a high voltage level. When the IGBT fully turned on and the collector was essentially at zero volts with respect to the emitter, the diode became conductive, thereby providing a signal to the gate control circuit. Thereafter, when the IGBT again turned off and the collector voltage increased, causing the diode to become non-conductive, the signal to the gate control circuit is altered. This is a rather rudimentary technique that simply provides a binary signal to the gate control circuitry indicating the on/off state of the transistor.
Prior devices avoided potential damage by switching the IGBT slowly, however, such an approach affects the rate at which the load current was controlled, which could result in energy losses. To minimize overall power dissipation, it is desirable to be able to switch the IGBT as fast as possible, without exceeding the maximum collector-emitter voltage rating of the IGBT. Achieving this operation requires more comprehensive information regarding the IGBT operation than can be provided by an implementation that uses only a sensing diode to monitor the IGBT device operation.
Because the collector voltage generally has values much higher than can be directly applied to circuits built with standard integrated circuit technologies, it is desirable to accurately scale this voltage to a level compatible with those of an integrated circuit connected to the device. Because it is desirable to avoid DC leakage paths, existing techniques such as using a resistor divider are not suitable to achieve a compatible voltage level.
Therefore, a monitor is desired that provides more detailed information about the status of the IGBT and in particular the collector voltage.